1. Technical Field
The present disclosure relates generally to a power drive system of light-emitting diode strings, and more particularly to a power drive system of light-emitting diode strings with a current-sharing function.
2. Description of Related Art
For many years, the light-emitting diodes (LEDs) play an important role in the backlight of portable electronic products. In the lighting application, LEDs are the most crucial components in the solid-state lighting industry. The advantages of LEDs include: energy saving, long life-span, free of maintenance, long life-span, and so on. In addition, a well-matched driving circuit for driving LEDs is very necessary in whether the lighting, the backlight, or the display fields. Especially to deserve to be mentioned, the backlight module is an important apparatus for the flat panel display. The backlight module determines the display quality of the flat panel display because of the reliability and stability of the LEDs.
However, the voltage of driving each LED string is not the same because forward voltage of each LED is not exactly identical so that drive currents flowing through the LED strings are not balanced. Accordingly, illumination of the LED string increased linearly with drive current is not uniform so as to reduce luminous efficiency of LED strings. Hence, in order to increase current balance capability between LED strings to improve the display quality of the flat panel display, various current-sharing circuits for LED strings are to come with the tide of fashion.
Reference is made to FIG. 1 which is a schematic circuit block diagram of a prior art drive system of light-emitting diode (LED) strings. The drive system of LED strings is supplied by an external AC power source Vac, and the external AC power source Vac is converted into a DC voltage Vbus by a rectifier 10A. In the drive system, a plurality of DC-to-DC regulators are provided to correspondingly control drive currents flowing through the LED strings. More specifically, the number of DC-to-DC regulators is identical to that of LED strings, and each LED string is electrically connected to one DC-to-DC regulator. Each DC-to-DC regulator receives the DC voltage Vbus outputted from the rectifier 10A and correspondingly provides the required LED drive voltage to the LED string. That is, the DC-to-DC regulators 21A-24A output LED drive voltages VLED1-VLED4 to correspondingly drive the LED strings. In addition, each DC-to-DC regulator detects the feedback current from the corresponding LED string and adjusts the LED drive voltage for driving the LED string, thus controlling the drive current flowing through the LED string.
Especially, in order to overcome the problem of different drive currents flowing through the LED strings because of the voltage difference between LED strings, the current-sharing mechanism is usually introduced by the DC-to-DC regulators to implement uniform illumination of the LED strings. Each LED string is required to use a DC-to-DC regulator so as to implement the current-sharing function, however, the circuit costs are significantly increased and the size of the power drive system is unfavorable.
Accordingly, it is desirable to provide a power drive system of LED strings to use only one current control IC to control total current flowing through the LED strings, and passive components, such as capacitors and chokes are used to implement the current-sharing function, thus simplifying circuit designs, significantly reducing circuit costs, and increasing adaptive operation to the power drive system.